1. Field of the Invention
The present invention relates generally to power electronics and more particularly to the control of power supply devices.
2. Description of the Related Art
Power supply devices usually generate, transmit, and distribute power at a fixed switching frequency and output voltage depending upon the application. Most power supplies typically use a converter built on a matrix of semiconductor switches to deliver power at the desired frequency and voltage. One particular class of converters used in high frequency applications is the resonant converter, which is formed using LC resonant tank circuits. The resonant circuit is driven with square waves of current or voltage, and by adjusting the switching frequency around the resonant point, the voltage across the resonant components can be adjusted. By rectifying the voltage across the resonant elements, a regulated DC output voltage is obtained which can be different than the input DC voltage to the converter. Other advantages are that the resonant converter operates in a zero-voltage or a zero-current switching mode and reduces electromagnetic interference.
While resonant converters exhibit the above advantages, they suffer from the disadvantage of being difficult to control because of the non-linearity associated with its resonant elements and rectifier circuitry. One particular type of approach to controlling a resonant converter has been to use a linear proportional-integral-derivative (PID) controller. However, the linear controller decreases the electrical performance of the resonant converter significantly. Other control approaches have used non-linear controllers. Examples of non-linear controllers are shown in Oruganti et al., Resonant Power Processors, Part II, IEEE Industrial Applications Society, 1461-1471 (1984); King et al. Inherent Overload Protection for the Series Resonant Converter, IEEE Transactions on Aerospace Electronic Systems, 820-830 (1983); Ranganathan et al., A Regulated DC-DC Voltage Source Converter Using A High Frequency Link, IEEE Industrial Applications Society, 279-287 (1982); Sanders et al., Generalized Averaging Method For Power Conversion Circuits, IEEE Power Electronics Specialists Conference, 273-290 (1989); Schwarz, An Improved Method of Resonant Current Pulse Modulation For Power Converters, IEEE Power Electronics Specialists Conference, 194-204 (1975). The above non-linear controllers are complex, require expensive sensors for operation, and have wide variations in stability margin, overshoot, rise time, output ripple, and transient response.